1. Field of the Invention
The present invention relates to dielectric ceramics, methods for producing the dielectric ceramics, and monolithic ceramic capacitors. The present invention particularly relates to an improvement in thickness reduction for dielectric ceramic layers for use in monolithic ceramic capacitors.
2. Description of the Related Art
Monolithic ceramic capacitors include capacitor bodies including a plurality of stacked dielectric ceramic layers and a plurality of internal electrodes extending between the dielectric ceramic layers. External electrodes are disposed on end surfaces of the capacitor body that are opposed to each other. The external electrode electrically connects the internal electrodes to each other. Some of the internal electrodes are electrically connected to one of the external electrodes and the other internal electrodes are electrically connected to the other one. The internal electrodes electrically connected to one of the external electrodes and the internal electrodes electrically connected to the other one are alternately arranged in the stacking direction of the dielectric ceramic layers.
The conductive material contained in the internal electrodes is usually Ni for cost reduction. In the manufacture of the monolithic ceramic capacitors, the capacitor bodies are fired such that the dielectric ceramic layers are sintered. The capacitor bodies need to be fired at a time when the internal electrodes are arranged in the capacitor bodies. Ni, which is contained in the internal electrodes, is a base metal and therefore the capacitor bodies need to be fired in reducing atmospheres.
The dielectric ceramic contained in the dielectric ceramic layers is usually BaTiO3, which has a high dielectric constant.
Thin dielectric ceramic layers are used to manufacture monolithic ceramic capacitors with high capacitance per unit volume.
It is effective to use the thin dielectric ceramic layers in combination with thin internal electrodes. However, the thin internal electrodes are likely to be spheroidized while being fired in reducing atmospheres and therefore are readily broken. In order to avoid such a problem, the thin dielectric ceramic layers can be sintered by low-temperature firing. Adding sintering aids containing, for example, SiO2, to ceramic materials is effective to allow the ceramic materials to be sintered at low temperature. Japanese Unexamined Patent Application Publication No. 2001-89231 (hereinafter referred to as Patent Document 1) discloses that lithium is effective in achieving low-temperature sintering.
In particular, Patent Document 1 discloses a dielectric ceramic composition containing a lithium compound and a major component containing 89% to 97% barium titanate in terms of BaTiO3, 0.1% to 10% yttrium oxide in terms of Y2O3, 0.1% to 7% magnesium oxide in terms of MgO, 0.01% to 0.3% vanadium oxide in terms of V2O5, 0.5% or less manganese oxide in terms of MnO, and 0.5% to 7% barium calcium silicate in terms of (Ba, Ca) SiO3, on a molar basis. The content of the lithium compound is 0.01 to 5.0 weight percent in terms of Li2O with respect to 100 mole percent of the major component.
Patent Document 1 indicates that lithium in the dielectric ceramic composition acts as a sintering aid and is involved in enhancing the temperature coefficient of dielectric constant of the dielectric ceramic composition.
On the other hand, compact monolithic ceramic capacitors are increasingly being demanded and therefore dielectric ceramic layers with a thickness of less than about 1 μm are demanded. The electric field applied to a dielectric ceramic layer increases with a reduction in the thickness of the dielectric ceramic layer. Therefore, in order to cope with the above demand, dielectric ceramics contained in the dielectric ceramic layers need to have good insulating properties and life properties. However, there is a problem in that the use of the dielectric ceramic composition disclosed in Patent Document 1 is not effective in achieving sufficient life properties.